Film thickness constraints for manufacturable strained silicon CMOS

This paper studies the effect of the strained silicon thickness on the characteristics of strained silicon MOSFETs on SiGe virtual substrates. NMOSFETs were fabricated on strained silicon substrates with various strained silicon thicknesses, both above and below the strained silicon critical thickness. The low field electron mobility and subthreshold characteristics of the devices were measured. Low field electron mobility is increased by about 1.8 times on all wafers and is not significantly degraded on any of the samples, even for a strained silicon thickness far greater than the critical thickness. From the subthreshold characteristics, however, it is shown that the off-state leakage current is greatly increased for the devices on the wafers with a strained silicon thickness that exceeds the critical thickness. The mechanism of the leakage was examined by using photon emission microscopy. Strong evidence is shown that the leakage mechanism is source/drain electrical shorting caused by enhanced dopant diffusion near misfit dislocations.